1. Field of the Invention
This invention relates to an improved process for recovering crude tall oil from tall oil soap. More particularly, this invention relates to a process for the acidification of tall oil soaps with carbon dioxide in the presence of a water-immiscible solvent.
2. The Prior Art
Tall oil is obtained from the black liquor of the alkaline digestion of coniferous wood, most notably the kraft process. The black liquor is concentrated and allowed to settle, the tall oil soap-containing portion at pH about 12 rises as a top layer, and is skimmed off. The black liquor soap skimmings recovered from the kraft process are reacted with a strong polar acid, usually sulfuric acid, to reduce the pH to about 3.5, and thereby convert the crude tall oil soaps to their free-acid form.
Upon acidification of the tall oil soaps, the reaction mixture is allowed to settle and forms three layers -- a crude tall oil layer, a lignin layer and a sulfate-brine layer. The sulfate-brine layer and lignin layers are returned to the pulp mill to recover the chemicals for use in the kraft pulping process. The crude tall oil layer is drawn off and placed in storage. The yield of crude tall oil varies somewhat depending upon the source of wood and the details of its processing, but the average recovery is about 1% based on the weight of wood pulped. The crude tall oil is then fractionally distilled to obtain approximately equal portions of rosin acids and fatty acids and to remove the greater portion of the odor and color-forming constituents.
Many pulp mills in recent years have been forced to eliminate effluent streams containing excessive amounts of sulfur compounds; and thus, the return of the sulfate-brine layer to the pulp mill may cause an intolerable build up of sulfur-containing chemicals in the pulp mill. Sulfuric acid acidification of tall oil soap generates more sulfates than the pulp mill's equilibrium chemical balance requires. In a truly closed mill system, sulfuric acid recovery of tall oil would raise the sulfidity above the levels dictated by good pulping practice if all the sulfates were returned to the pulp mill recovery system. Therefore, it has become necessary in tall oil refineries related to paper mills to eliminate or substantially lessen the return of sulfates to the paper mill.
As stated, commercially recovered tall oil is acidified using sulfuric acid; and numerous attempts to acidify with other acids have been attempted. To liberate completely the crude tall oil from the soap in one step, it has been necessary to acidify by lowering to about pH 3.5. Therefore, attempts to use other acids, such as carbonic acid or CO.sub.2 or a gas stream containing CO.sub.2, have met with only partial success because of the inability to reach the lower pH range.
One process which does show promise for recovery of crude tall oil using CO.sub.2 acidification is set forth in U.S. Pat. No. 3,901,869. However, because the process of U.S. Pat. No. 3,901,869 uses water and carbon dioxide when the mixture is acidified to a pH of about 7-8, only a portion of the tall oil soap is converted to the desired free acids.
It is thus the general object of this invention to provide a process for recovering crude tall oil from tall oil soaps.
Another object of this invention is to provide a process for the total acidification of tall oil soaps with carbon dioxide and a solvent.
Other objects, features and advantages of this invention will be evident from the following detailed description.